The present invention relates to microelectronic packaging.
Electronic devices referred to as surface acoustic wave or “SAW” devices process electronic signals in the form of acoustical waves, i.e., minute mechanical vibrations transmitted within the device, typically on an exposed surface region of a mass of a crystalline material. SAW devices are used, for example, as frequency-selective filters and as mixers in analog signal processing. Among other applications, SAW devices are used in radio frequency circuits of cellular telephones and other portable electronic apparatus. SAW devices normally must be provided with a cover or “cap” overlying the acoustically-active region of the surface to protect the active surface from mechanical engagement with surrounding structures and from chemical reaction with the surrounding atmosphere. Likewise, certain micro-electromechanical devices and micro machines incorporate microscopic mechanical elements within an active region of the device. The active regions of these devices must be covered by caps to protect the micromechanical elements. Such devices typically are formed using techniques commonly employed to make conventional microelectronic devices, and are commonly referred to by the acronym “MEMS”.
Miniature SAW devices can be made in the form of a wafer formed from or incorporating an acoustically active material such as lithium niobate material. The wafer is treated to form a large number of SAW devices, and typically also is provided with electrically conductive contacts used to make electrical connections between the SAW device and other circuit elements. After such treatment, the wafer is severed to provide individual devices. SAW devices fabricated in wafer form have been provided with caps while still in wafer form, prior to severing. For example, as disclosed in U.S. Pat. No. 6,429,511, a cover wafer formed from a material such as silicon can be treated to form a large number of hollow projections and then bonded to the top surface of the active material wafer, with the hollow projections facing toward the active wafer. After bonding, the cover wafer is polished to remove the material of the cover wafer down to the projections. This leaves the projections in place as caps on the active material wafer, and thus forms a composite wafer with the active region of each SAW device covered by a cap.
Such a composite wafer can be severed to form individual units. The units obtained by severing such a wafer can be mounted on a substrate such as a chip carrier or circuit panel and electrically connected to conductors on the substrate by wire-bonding to the contacts on the active wafer after mounting, but this requires that the caps have holes of a size sufficient to accommodate the wire bonding process. This increases the area of active wafer required to form each unit, requires additional operations and results in an assembly considerably larger than the unit itself.
In another alternative disclosed by the '511 patent, terminals can be formed on the top surfaces of the caps and electrically connected to the contacts on the active wafer prior to severance as, for example, by metallic vias formed in the cover wafer prior to assembly. However, formation of terminals on the caps and vias for connecting the terminals to the contacts on the active wafer requires a relatively complex series of steps.
Similar problems occur in providing terminals for MEMS devices. For these and other reasons, further improvements in processes and structures for SAW, MEMS and other capped devices would be desirable.